Actually, there are examples of "one off" events in early human history, such as the migration out of Africa by a subset of the ancestral human population some 50,000 years ago.
According to Nicolas Wade's fascinating book "Before the Dawn" (yes, the same Nicolas Wade from TFA), all the genetic evidence points to a single band of maybe 150 people leaving the rest of the ancestral human population behind in Africa, and populating all the rest of the world. Of course, the natural question is, why didn't other waves follow them in all the millennia since then?
The answer is, in part, that the first migrants already blocked the exits. The original departure from Africa was less a migration than it was an expansion... individuals tended to live in roughly the area they were born, and it was only the ever-growing population numbers that drove the advancing wave of modern humans through Asia and Europe generation after generation. The modern humans had a strong advantage (probably language) over the archaic hominids already occupying the new lands, but the human population in Africa had no such advantage over their brethren once the first wave spread out past the Red Sea. Hence, the migration out of Africa appears to have been a one-time event of the type you so quickly derided as nonsense.
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